Operation to produce an intermediate product for a crucible for a LUWPL is as follows: a) a body 2 is preheated and placed on a support, with its bore concentric with a tube 4 supported in a chuck and connected to a inflation means; b) the tube is heated with the chuck being rotated for evenness of heating; c) when the temperature of the tube is detected to be the softening point of the quartz of the tube, its rotation is stopped and it is advanced into a bore 3 in the body 2; d) advance is stopped when the distal, sealed end is detected to have reached a determined protrusion 18; e) simultaneously with the advance being stopped, inflation gas is admitted into the tube, to inflate it albeit it marginally, and bring its outer surface 5 into intimate contact with the surface 6 of the bore 4.